This invention relates to a mercury remediation method and apparatus, and more particularly to a five-step method for reducing mercury levels in water to a nanogram per liter level.
Mercury is a globally ubiquitous, naturally occurring element that is released into the atmosphere as a result of natural events such as volcanic eruptions and forest fires and through human or industrial activities such as the combustion of fossil fuels primarily in electric power production, but also emitted through process operations such as Portland Cement production, municipal and medical solid waste incineration, chlor-alkali production and mining operations. It is estimated that the proportion of these releases is 30% by natural events, and 70% through human or anthropogenic activities.
Mercury is toxic to humans when it is ingested or inhaled. Methylmercury (CH3Hg+) is formed when elemental mercury is metabolized in sediment or soils by microbial activity. It is of particular concern because methylmercury tends to accumulate within the food chain, making it a persistent bioaccumulative pollutant. Concentrations of methylmercury in fish can be on the order of a million times the methylmercury concentration in the sediment of the surrounding water. The U.S. EPA and the U.S. Food and Drug Administration (FDA) have issued fish and shellfish consumption advisories particularly aimed at women who might become pregnant, nursing mothers, and young children because of the elevated risks to the child or fetus of neurologic impairment during early development. The Mount Sinai School of Medicine's Center for Children's Health and the Environment published a 2005 report1 that estimated that the economic consequence of the lost productivity in the U.S. due to methyl mercury toxicity is $8.7 billion per year (in 2000 dollars).
To address these health risks, the U.S. EPA has established the wastewater mercury concentration limit of 1.3 nanograms per liter for wastewater that is discharged into the Great Lakes basin (40 CFR 132.6, Table 4). It has also established the test method used to determine mercury concentration in water, namely, U.S. EPA Method 1631, Revision E: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry.2 
Mercury bearing wastewater and air scrubber water will invariably contain particulate material and suspended (undissolved) solids. This presents a significant challenge to the removal of mercury because of the strong attraction mercury has for an undissolved solid particle. Particle-bound mercury is unavailable to react with traditional chemical precipitation technologies such as sodium diethyldithiocarbamate (also called dithiocarbamate, or DTC for short) or to react with precipitation-based, fixed bed adsorption removal systems such as a sorbent trap (for example, a sulfur impregnated activated carbon), or various types of ion exchange resins that are commercially available and marketed for mercury control. These methods rely on mercury physically reacting with the chemical precipitant for the mercury removal. However, if the mercury is bound to a particle, the particle will interfere with the chemical precipitant's ability to physically react with and chemically bond to the mercury, which undermines the strategy of mercury removal methods.
As a result, particle-bound Hg is traditionally removed with physical filter type liquid/solids separation devices such as a sand or anthracite media filter or a membrane filter. The very small mercury-bound particle size coupled with the very low Hg discharge concentration limit requires that extremely fine particle size removal performance be accomplished. This very fine particle size removal performance is most commonly accomplished with costly membrane technologies such as ultrafiltration (with membrane pore sizes ranging from 300 nm to 10,000 nm), nanofiltration (˜90 nm-800 nm pore sizes) or hyperfiltration (also called reverse osmosis, or RO, ˜0.1-1 nm pore sizes).
These membrane systems are expensive both from a capital and an operational standpoint. The membranes are expensive to buy and have a relatively short life-cycle, requiring continuous membrane element replacement at considerable cost. Membrane systems require a lot of space. The operational costs include significant wastewater (feedwater) pretreatment requirements, off-line membrane cleaning necessities that include equipment and chemicals, and the need to manage the wastewater generated by the membrane system itself (membrane cleaning solution and wash-water). Despite these significant cost requirements, a membrane system still is not designed nor intended to reduce the dissolved Hg concentration.
The membrane-based system must either precede or follow a traditional chemical precipitation type system using dithiocarbamate precipitant or a precipitation-based, fixed bed adsorption removal system that can handle the dissolved Hg. The membrane-based system alone is estimated to be 10 to 40 times more expensive to purchase and operate than for the mercury remediation method and apparatus described in this patent. Add to it the cost of the chemical precipitation system or a mercury-precipitate fixed-bed system, and the membrane remediation approach becomes further cost prohibitive.
Nguyen in U.S. Pat. No. 4,160,730 teaches a method for removing and recovering mercury from aqueous media containing undissolved solids using oxidation, reduction, and aeration. The method is best suited for removing inorganic ionic mercury from chloralkali plant effluent. Hayashi, et al. in U.S. Pat. No. 4,599,177 teach a method for removing mercury, and other heavy metals, from incinerator waste gas by a process that includes washing the waste gas, treating the resulting wastewater with a reducing agent and then subjecting the resulting treated wastewater to stripping treatment in the presence of ferrous ions. Hamilton et al. in U.S. Pat. No. 6,521,131 teaches a method of removing mercury-complexing material from wastewater by strong oxidation followed by mercury-selective absorbent material consisting of a crosslinked styrene-divinylbenzene substrate with dithiocarbamate groups bound thereto. From their tables, their method is capable of reducing the concentration of mercury in contaminated groundwater to the nanogram per liter level, however for wastewater and incinerator scrubber waste only mercury concentrations in the hundreds of nanograms per liter were attained. Broderick in U.S. Pat. No. 6,942,840 describes a method for converting vapor-phase mercury from a gas stream into a liquid in the presence of a precipitant which would then immediately convert the dissolved mercury into a precipitated form.